1. Field of the Invention
The invention relates to the field of endovascular devices and in particular to devices used to control blood heating for selective heat treatment of tissue, such as for cancer treatment.
2. Description of Prior Art
The use of hyperthermia for preferential killing of malignant cancer cells is well known. While cancer cells themselves may not be more heat sensitive than normal cells, their environments render them so, namely cancer tissues are normally subject to nutritional deprivation, low pH and chronic hypoxia. It is also apparent that the use of heat has a synergistic effect with x-ray radiation and with cytotoxicity of many anti cancer drugs. Because of the cooling action of blood flow, heating of solid tumors and other vascular lesions can be difficult to arrange and varies with the variable blood flow characteristics.
Blood flow plays an extremely important role in determining the response of tumors to localized hyperthermia. First of all, blood flow can dramatically affect temperature distributions. This is not surprising because the removal of heat from the tumor volume is primarily carried out by blood flow and therefore nonuniform blood flow invariably results in nonuniform temperature distributions. Furthermore, blood flow does not remain constant during heat treatment. In normal tissue, the body increases flow within the heated volume in an attempt to affect cooling. Some tumors, although not all, behave similarly. In other tumors, this homeostatic control response is either reduced or missing altogether. After extended periods of heating of greater than 30 minutes and particularly at temperatures of 43.degree. C. or higher, blood flow in some murine tumors stops almost entirely. For these reasons, temperature monitoring during heating is essential. Finally, blood flow to a large extent determines the availability of tumor cells of oxygen, nutrients as well as drugs during chemotherapy. All these can influence the response of cells within tumors to treatments.
The killing of cells is highly effective with respect to temperature. The change of even one degree C can cause appreciable differences in cell killing rates. However, when heat alone is relied upon for treatment, either the treatment temperature must be so high that sufficient cell killing occurs even in the cold spots within the cancerous tissue or the temperature uniformity within the heating tumor must be maintained at a predetermined magnitude to within about a degree C. Each of these approaches entailed great technical difficulties.
In those tumors where blood flow is highly nonuniform, avoidance of cold spots becomes a particularly difficult problem. If any major blood flow vessels traverse the lesion, then uniform heating is probably impossible. Blood flow in those vessels is so great that in their vicinity some degree of cooling is inevitable. In addition to blood flow, the mode of heating can create nonuniform temperature distributions.
Some cancers are so virulent that survival of one or at most a few of these cells can lead to regrowth of the tumor and hence failure of the treatment. Obviously, then, if the heated tumor contains even small volumes of inadequately heated sections, its eradication by hyperthermia becomes highly unlikely, particularly when it is kept in mind that one gram of tissue contains approximately tens of millions of cells.
Methods of inducing hyperthermia can be divided into two broad categories. In regional heating, energy is deposited in general volume containing the tumor. Some special characteristics of the tumor, such as reduced blood flow compared that in the surrounding normal tissues, is relied upon for the tumor to reach the therapeutic temperature while the normal tissues remain cool.
In focused heating, the energy is deposited directly within the tumor. Less reliance is placed on the differences between the tumor and normal tissues, but the special definition of the energy deposition must be much more precise. A number of methods of focused heating have been suggested including noninvasive technique of focused ultrasound and invasive techniques of interstitial radio frequency currents, interstitial microwave antennas, and use of lossy magnetic material to preferentially absorb energy from an external electromagnetic field.
The implantation of lossy magnetic materials, which are then localized and heated in place, is limited by the magnetic characteristics of the materials, the ability to secure them into a localized position and the long term effects of their presence, since in most cases they cannot be removed or are only very slowly dispersed from the implantation site.
Interstitial implantation of heated fluid has also been utilized through delivery catheters. However, the amount of heated fluid that can be delivered to the tumor site as well as the length of time in which the tumor can be heated is limited since the capacity for fluid absorption in any tissue, particularly denser tissue such as brain tissue, is necessarily limited.
If ultrasound is used to deposit energy within the tumor volume, shielding by bony structure may prevent heating in some volumes. Bones also reflect ultrasound, thereby also contributing to nonuniformities. Bones are reasonably good conductors of heat and therefore near bones, areas of low temperature may be encountered.
Nonuniformity of either electric field strength of current density can also result in the occurrence of cold spots when dielectric heating techniques are used. These result from nonuniform power deposition patterns, or from nonuniform absorption characteristics of the tissue irradiated.
Therefore, what is needed is some means which is not subject to the defects of the prior art but which will allow localized heating in both degree and time for the effective therapeutic treatment of tissue.